The field of the invention is magnetic resonance spectroscopy ("MRS") and imaging ("MRI"). More particularly, the invention relates to the acquisition of spectra and images from multiple regions of a subject.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins after the excitation signal B.sub.1 is terminated and this signal may be received and processed to provide a spectrum.
Magnetic field gradients (G.sub.x, G.sub.y and G.sub.z) are employed to localize the region in the object from which the spectra or images are obtained. In point resolved spectroscopy (PRESS), for example, gradient fields are used in combination with selective RF pulses to acquire spectra from a voxel located at the intersection of three orthogonal slices. On the other hand, in spectroscopic imaging (SI) one gradient field is used in combination with a selective RF pulse to excite spins in a slice, and the other two gradients are used to phase encode the acquired NMR spectra and localize signals to voxels in the slice.
Recent MRS applications employ long repetition rates (TR) to allow full recovery of the longitudinal spin magnetization between acquisitions. Since it is common to average from 64 to 256 acquisitions from the same location, the time needed to acquire spectra from a single location is quite long. The time needed to acquire successive MR spectra from different locations in the subject thus becomes prohibitive.
In conventional MRI, this problem is solved by interleaving acquisitions from different slice locations. While the longitudinal magnetization in one slice is recovering, image data is acquired from the remaining separate slices. Thus, during the time needed to scan one slice, image data is acquired for many slices. This solution is possible in conventional MRI because the same pulse sequence is used to scan all of the interleaved slices--only the slice location is changed. A single adjustment of homogeneity, pulse power and frequency is made for all slices. In MRS and in chemical shift sensitive MRI (e.g. chemical shift selective fat saturation) a single static adjustment of these parameters is often unacceptable.
Depending on the particular spectrum being obtained, a number of spectroscopy pulse sequence parameters are usually adjusted to obtain the desired information from a location in a subject. For example, the size of the voxel may be different, shim coil corrections may be different, and water or fat suppression preparatory pulses may be different. Consequently, it is not possible to simply interleave spectroscopy acquisitions as is done with conventional MRI.